The ultra-rare lysosomal diseases collectively affect 1 in 3,000 individuals and are responsible for significant morbidity and disability. The Lysosomal Disease Network (LDN) has created a community of clinical investigators, patient advocacy groups (PAG), and other interested parties, to become a synergistic research and educational consortium advocating advancement of treatment for these diseases. In the past three decades, lysosomal diseases have been a test bed for some of the most innovative therapeutic modalities. In the past 4 years of NIH funding, the LDN has accelerated knowledge acquisition in the field - with 33 MyNCBI cited publications, and 12 more in press/review/preparation-and furthered the development of therapeutic options. For the next 5 years, this proposal describes 9 longitudinal studies of natural history and/or treatment, and 5 pilot studies for novel ideas under the central theme of discovery in ultra-orphan diseases. Because central nervous system (CNS) disease is the most difficult to treat and measure, there is a major emphasis on quantitative analysis of CNS structure, function and biomarkers for relevant conditions: mucopolysaccharidoses (MPS), mucolipidosis IV, Batten disease, gangliosidoses (Tay-Sachs, Sandhoff and GM1 gangliosidosis diseases), globoid cell leukodystrophy. Projects will (a) evaluate immune modulatory factors affecting treatment response in Pompe disease; (b) assess bone disease in MPS, (c) correlate renal structure and function in Fabry disease; (d) search for undiagnosed Fabry disease in high-risk populations; (e) determine outcomes of newborn screening for Krabbe disease; and (f) shorten the diagnostic odyssey. Productivity is assured by a new performance-based model, with proposals initially vetted for rigorous statistical significance, and reimbursement to investigators governed by DMCC data submission. In addition, this network: (a) supports two postdoctoral fellows each year; (b) organizes a scientific meeting WORLD Symposium published annually (February, Molecular Genetics & Metabolism); and (c) a didactic course for experts, Lysosomes 101. Global communication is provided by a list-serve with more than 3,000 subscribers, webinars, and the website LysosomalDiseaseNetwork.org.